System for controlling the traction slip of a vehicle

ABSTRACT

A method for controlling the traction slip of a vehicle on a roadway with sidewise different coefficients of friction includes the following steps: identifying a driving situation on a roadway with sidewise different coefficients of friction and, when the driving situation is identified and traction slip is encountered on both wheels, decreasing the brake pressure of the driven wheel on the low coefficient-of-friction side. A device for controlling the traction slip of a vehicle on a roadway with sidewise different coefficients of friction includes a determining device for determining a driving situation on a roadway with sidewise different coefficients of friction, and a brake actuation control which decreases the brake pressure of the driven wheel on the low coefficient-of-friction side when the driving situation is identified and traction slip is encountered on both wheels.

TECHNICAL FIELD

The present invention generally relates to vehicle stability control,and more particularly relates to a method and a device for controllingthe traction slip of a vehicle on a roadway with sidewise differentcoefficients of friction.

BACKGROUND OF THE INVENTION

Traction slip principally means that one or more vehicle wheels rotatefaster than a wheel speed that corresponds to the current driving speedof the vehicle. This is mostly the case when the driving torque exceedsthe torque which can be transmitted between the wheels and the roadwaydue to the frictional conditions prevailing between them.

FIG. 1 is a schematic view of a vehicle from the top. Reference numeral11 relates to the left front wheel, 12 to the right front wheel, 13 tothe right rear wheel, and 14 to the left rear wheel. When the vehiclehas a front-wheel drive, traction slip can occur at wheels 11, 12. Toreduce traction slip, it is either possible to reduce the engine torque,or brake pressure can be built up to decelerate the spinning wheels. Thelatter action is chosen especially in the presence of low rotationalspeeds when the reduction of the engine torque is scarcely possible.

Comparatively unfavorable driving situations may exist. For example, thesides of a roadway may be split with respect to coefficients offriction, e.g. in such a way that the right-hand wheels run on a lowcoefficient of friction (roadway is wet, full of sand, slippery), whilethe left-hand wheels run on a high coefficient of friction (roadway isdry). In FIG. 1, this is represented by hatched lines 17 which is meantto show the roadway areas having a low coefficient of friction. Withouttraction slip control, the case may occur that the driven wheel on a lowcoefficient of friction (e.g. 12) spins, while the wheel on a highcoefficient of friction is hardly driven due to the effect of thedifferential. Traction is poor then and destabilizing.

Wheel-related traction slip control systems are known in the art (forexample BTCS=‘brake traction control system’) which control the wheelslip on each individual wheel. Sometimes high traction slip values arerequired in difficult situations during starting-to-drive maneuverswhich must not be suppressed in terms of control in order not to renderstarting to drive impossible. If in such unfavorable situations, forexample, when driving uphill on split coefficients of friction, tractionslip develops on the high coefficient-of-friction side as well, thebuild-up of brake pressure will impair the comfort and, possibly, alsocause loss in traction.

On the other hand, there are traction slip control systems which controlthe traction slip on one wheel only, but in doing so also consider thebehavior of another wheel (for example, EDL=‘electronic differentiallock’) under certain conditions. The objective of these systems in termsof control, especially in the presence of major differences incoefficients of friction, is to develop brake pressure only on the sidehaving a low coefficient of friction, in order to make the entire engineoutput available for traction. Admittedly, these systems control thebrake pressure on the low coefficient-of-friction side also independence on the behavior of the driven wheel on the highcoefficient-of-friction side. However, when traction slip develops onthe high coefficient-of-friction side, the shortcoming involved is lossin traction due to a considerable reduction of the supporting torque anda reduced comfort due to a high amount of traction slip on the lowcoefficient-of-friction side. No intervention is made on roadways havingthe same coefficients of friction on both sides and, therefore,approximately the same amount of traction slip on all driven wheels.

An object of the present invention is to provide a method and a devicefor controlling the traction slip of a vehicle on a roadway withsidewise different coefficients of friction which permit maximizingtraction and involve least possible impairment of comfort.

Initially, the driving situation on a roadway with sidewise differentcoefficients of friction is identified. Identification may be performed,for example, by comparing the amounts of slip on the wheels of thedriven axle. When the amounts of slip are obviously different, thisindicates a driving situation with split coefficients of friction.Instead of the amounts of slip, however, the respective wheel speeds mayalso be chosen for consideration (because the amounts of slip arerespectively calculated as a difference between the wheel speed and thevehicle reference speed).

To counteract loss in traction, the brake pressure on the driven wheelon the low coefficient-of-friction side may be decreased when tractionslip occurs on both wheels of the driven axle. Although this causeshigher traction slip on the low coefficient-of-friction side, it alsoproduces (due to the differential) a lower slip amount on the highcoefficient-of-friction side, in the most favorable case, it effects newgrip of this wheel and, hence, an improved traction, a constant driveand, thus, driving comfort which is reduced only inconsiderably.

A traction slip control device includes an identification device foridentifying the split coefficients of friction on the roadway and abrake actuation control which reduces the brake pressure on the lowcoefficient-of-friction side in the event of split coefficients offriction and traction slip that prevails on both sides. The brakeactuation control may be a part of a comprehensive brake control systemwhich fulfills different control objectives such as that of an anti-locksystem or electronic stability optimization. In particular, the tractionslip control according to the present invention may be a part of a priorart traction slip control system. It is preferably configured as asupplement to traction slip control on each individual wheel. Thus, itmay e.g. add to the BTCS mentioned hereinabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic top view of a vehicle.

FIG. 2 is a schematic block diagram showing the components of tractionslip control which are essential for this invention.

FIG. 3 shows the identification device of FIG. 2.

FIG. 4 shows the brake actuation control of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of FIG. 2 shows a traction slip control which includes anidentification device 22 and a brake actuation control 24. In general,the components described hereinabove may be configured to implement themethod steps which have also been described, and method steps may beperformed corresponding to the individual components described.

Different tests are carried out in the identification device in order tobe able to positively recognize the situation, on the one hand, andprevent wrong identifications, on the other hand, and in order toidentify in addition the side with low coefficients of friction or theside with high coefficients of friction, in case of need.

In general, the traction slip control system can receive most differentinput signals 21. The signals may e.g. comprise the wheel signalsobtained from the wheel sensors. Apart therefrom, further sensor signalsor secondary signals may be received and evaluated, as the requirementmay be. To the extent provided, either direct sensor signals can beused, or conditioned (filtered) sensor signals or signals from a databus can be used. Such sensorbased signals are referred to by referencenumeral 21 irrespective of their origin.

The identification device 22 is shown in greater detail in FIG. 3. Itincludes a determining device 30 by which the highcoefficient-of-friction side or the low coefficient-of-friction side ofthe vehicle can be identified. The determining device 30 may e.g.comprise a comparison device 33 which permits comparing the slip amountsof the driven wheels. As one may a priori assume the same driving torqueon both wheels, that wheel can be considered to be a wheel on the sidewith the lower coefficient of friction that has the higher amount ofslip. Instead of the slip amounts of the driven wheels, the wheel speedsof these wheels may also be compared to each other. The identificationdevice produces an output signal which characterizes the side with thehigh or with the low coefficient of friction.

Further, there is provision of a checking device 32 in which differentchecks can be performed. Individual, several, or all of the checksmentioned hereinbelow can be made:

A check is made whether the slip of the driven wheel on the highcoefficient-of-friction side is above a first threshold value.

A check is made whether the slip of the driven wheel on the lowcoefficient-of-friction side is above a second threshold value.

A check is made whether the slip acceleration and/or the wheelacceleration of the driven wheel on the high coefficient-of-frictionside is above a positive threshold value and whether the slipacceleration and/or the wheel acceleration of the driven wheel on thelow coefficient-of-friction side are below a negative threshold value.

The brake pressures at the driven wheels are checked with respect totheir value and/or compared to one another.

Both driven wheels must exhibit slip because the problem to be overcomeby the present invention exists only then. The slip of the driven wheelon the high coefficient-of-friction side shall not become so high thatconventional traction slip control systems will intervene. Therefore,the first threshold value shall lie below an intervention threshold ofconventional traction slip control systems.

The slip acceleration or wheel acceleration of the highcoefficient-of-friction wheel is polled and checked with respect to apositive threshold value. When the values are negative, this indicatesdecrease of the slip or of the wheel speed of the highcoefficient-of-friction wheel. Intervention is not necessary in thiscase because automatic correction of the situation may be expected.

The slip acceleration or wheel acceleration of the lowcoefficient-of-friction wheel is also polled to prevent unnecessaryinterventions: when the acceleration is obviously negative, thisindicates an excessive deceleration of the low coefficient-of-frictionwheel so that a favorable result may be expected as a result of theintervention according to the present invention (pressure reduction onthe low coefficient-of-friction wheel).

The brake pressure on the low coefficient-of-friction wheel shall behigher than a minimum value. The related threshold value for the minimumbrake pressure, in the sense of a hysteresis, can be higher in theabsence of the driving situation than in its presence. Preferably, thedriving situation is only recognized when the brake pressure on the lowcoefficient-of-friction wheel is higher, at least by a defined amount,than the brake pressure on the high coefficient-of-friction wheel.

When the mentioned conditions prevail, the driving situation withsidewise different coefficients of friction can be identified, anddepending on the number of criteria further taken into account, theidentification also contains a judgment as to whether the envisagedintervention is appropriate or will be successful. When the checkingdevice 32 comes to a positive result in total, it will issue a signalwhich, along with the signal of the determining device 30, can be outputas a total signal 23. FIG. 3 shows two signal lines as an example.However, the identification device can also produce a signal 23 which ismore complex and provides e.g. quantitative specifications.

When the identification device 22 outputs an identifying signal 23, thebrake actuation control 24 will trigger brake pressure reduction on thelow coefficient-of-friction side. The brake actuation control 24 isdescribed in greater detail in FIG. 4. It comprises a nominal valuesoutput 40 and a control signal generation means 44. The nominal valuesoutput 40 can include a specification 41 for nominal pressure gradientsand a specification 42 for nominal pressures. For example, whenidentification took place, a defined negative nominal gradient for thebrake pressure of the wheel on the low coefficient-of-friction side maybe adjusted, and this gradient in turn can depend on the runningbehavior (slip, speed, acceleration) of the low coefficient-of-frictionwheel and/or the high coefficient-of-friction wheel. In the controlsignal generation means 44, actuating signals for the valves of thewheel brakes are generated according to the predetermined nominal values(nominal pressure, nominal gradient) and output as output signals 25and, among others, supplied to the valve block.

Identification device 22 and brake actuation control 24, in turn, mayreceive input signals 21, especially wheel speeds, vehicle referencespeed, brake pressure signals, and similar signals. The signals can betaken directly or indirectly from sensors, a vehicle data bus, or models(for example, brake pressure models).

In the traction slip control according to the present invention,traction slip on the high coefficient-of-friction side will exist for acomparatively short time only so that the pressure on the lowcoefficient-of-friction side will also be reduced for a comparativelyshort period only. The increase in traction slip which is the result hasonly so low an amount that driving comfort will not be reduced thereby.

When traction slip on the high coefficient-of-friction side exceeds athreshold value despite the measures taken, the brake slip can becontrolled or regulated individually on this wheel. Thus, a higheramount of traction slip may occur temporarily on the highcoefficient-of-friction side also in processes such as uphill drivingmaneuvers e.g. due to inertia moments in the drive train or due to acomparatively low coefficient of friction on the highcoefficient-of-friction side. This situation will not satisfy theabove-mentioned criteria for a pressure reduction on the lowcoefficient-of-friction wheel so that pressure reduction is notinitiated there.

What is claimed is:
 1. Method of controlling a traction slip of avehicle on a roadway with sidewise different coefficients of friction,comprising the steps of: identifying a driving situation on a roadwaywith sidewise different coefficients of friction and, when the drivingsituation is identified and traction slip is encountered on both wheels,decreasing the brake pressure on the driven wheel on the lowcoefficient-of-friction side, further including the steps of:determining which vehicle side is the high coefficient-of-friction sideand which is the low coefficient-of-friction side, checking whether theslip of the driven wheel on the high coefficient-of-friction side isabove a first threshold value, checking whether the slip of the drivenwheel on the low coefficient-of-friction side is above a secondthreshold value, checking whether an acceleration of the traction slipor a wheel acceleration of the driven wheel on the highcoefficient-of-friction side is above a positive threshold value andwhether the acceleration of the traction slip or the wheel accelerationof the driven wheel on the low coefficient-of-friction side is below anegative threshold value, and checking the brake pressures on the twowheels or comparing the brake pressures one to the other, wherein whenchecking the brake pressures, the brake pressure on the lowcoefficient-of-friction side is checked as to whether it is above athird threshold value, and the brake pressure on the highcoefficient-of-friction side is checked as to whether it is below afourth threshold value.
 2. Method as claimed in claim 1, furtherincluding the step of comparing the amounts of slip or speeds of thedriven wheels when fixing the high or low coefficient-of-friction side,and wherein the side on which the higher amount of slip prevail, isdefined as the low coefficient-of-friction side.
 3. Method as claimed inclaim 1, wherein the first threshold value is set to be lower than athreshold value where traction slip control individually related to awheel intervenes.
 4. Method as claimed in claim 1, wherein whencomparing the brake pressures a check is made whether the brake pressureon the high coefficient-of-friction side is below the brake pressure onthe low coefficient-of-friction side at least by a defined amount. 5.Method as claimed in claim 1, further including using filtered values aswheel slip values.
 6. Method as claimed in claim 1, further includingthe step of decreasing the brake pressure according to a definablegradient.
 7. Method as claimed in claim 6, wherein the gradient isdetermined according to the slip or the acceleration of the tractionslip of the wheel on the high coefficient-of-friction side or the wheelon the low coefficient-of-function side.
 8. Method as claimed in claim1, wherein the method is performed in addition to a traction slipcontrol method which acts on the individual wheel, wherein the tractionslip on the individual wheel is compared to a threshold which isresponsive to the vehicle speed, and is changed by engine torquereduction or a wheel-individual brake pressure increase.
 9. Device forcontrolling a traction slip of a vehicle on a roadway with sidewisedifferent coefficients of friction, comprising: an identification devicefor identifying a driving situation on a roadway with sidewise differentcoefficients of friction, a brake actuation control which decreases thebrake pressure of the driven wheel on the low coefficient-of-frictionside when the driving situation is identified and traction slip isencountered on both wheels, wherein the identification device includesthe following components: a determining device for determining whichvehicle side is the high coefficient-of-friction side and which is thelow coefficient-of-friction side, and a checking device for checkingwhether the slip of the driven wheel on the high coefficient-of-frictionside is above a first threshold value, whether the slip of the drivenwheel on the low coefficient-of-friction side is above a secondthreshold value, whether an acceleration of the traction slip or a wheelacceleration of the driven wheel on the high coefficient-of-frictionside is above a positive threshold value, and whether the accelerationof the traction slip or the wheel acceleration of the driven wheel onthe low coefficient-of-friction side is below a negative thresholdvalue, and for checking the brake pressures on the two wheels orcomparing the brake pressures one to the other, wherein the checkingdevice checks the brake pressure on the low coefficient-of-friction sideas to whether it is above a third threshold value, and checks the brakepressure on the high coefficient-of-friction side as to whether it isbelow a fourth threshold value.
 10. Device as claimed in claim 9,wherein the determining device includes a comparison device forcomparing the amounts of slip or speeds of the driven wheels, and thatside on which the higher amount of slip or the higher speed prevails isdefined as the low coefficient-of-friction side.
 11. Device as claimedin claim 9, wherein the first threshold value is set to be lower than athreshold value where traction slip control individually related to awheel intervenes.
 12. Device as claimed in claim 9, wherein that whencomparing the brake pressures a check is made as to whether the brakepressure on the high coefficient-of-friction side is below the brakepressure on the low coefficient-of-friction side at least by a definedamount.
 13. Device as claimed in claim 9, wherein filtered values areused as wheel slip values.
 14. Device as claimed in claim 9, wherein thebrake actuation control includes a preset values output for predefininga nominal gradient for brake pressure decrease.
 15. Device as claimed inclaim 14, wherein the preset values output determines the gradientaccording to the slip or the acceleration of the traction slip of thewheel on the high coefficient-of-friction side or the wheel on the lowcoefficient-of-friction side.
 16. Device as claimed in claim 9, furtherincluding means for comparing the traction slip on the individual wheelto a threshold which is responsive to the vehicle speed, and is changedby engine torque reduction or wheel-individual brake pressure increase.17. Method of controlling a traction slip of a vehicle on a roadway withsidewise different coefficients of friction, comprising the steps of:identifying a driving situation on a roadway with sidewise differentcoefficients of friction and, when the driving situation is identifiedand traction slip is encountered on both wheels, decreasing the brakepressure on the driven wheel on the low coefficient-of-friction side,further including the steps of: determining which vehicle side is thehigh coefficient-of-friction side and which is the lowcoefficient-of-friction side, checking whether the slip of the drivenwheel on the high coefficient-of-friction side is above a firstthreshold value, checking whether the slip of the driven wheel on thelow coefficient-of-friction side is above a second threshold value,checking whether an acceleration of the traction slip or a wheelacceleration of the driven wheel on the high coefficient-of-frictionside is above a positive threshold value and whether the acceleration ofthe traction slip or the wheel acceleration of the driven wheel on thelow coefficient-of-fiction side is below a negative threshold value, andchecking the brake pressures on the two wheels or comparing the brakepressures one to the other, wherein when comparing the brake pressures acheck is made whether the brake pressure on the highcoefficient-of-friction side is below the brake pressure on the lowcoefficient-of-friction side at least by a defined amount.
 18. Method ofcontrolling a traction slip of a vehicle on a roadway with sidewisedifferent coefficients of friction, comprising the steps of: identifyinga driving situation on a roadway with sidewise different coefficients offriction and, when the driving situation is identified and traction slipis encountered on both wheels, decreasing the brake pressure on thedriven wheel on the low coefficient-of-friction side, further includingthe steps of: determining which vehicle side is the highcoefficient-of-friction side and which is the lowcoefficient-of-friction side, checking whether the slip of the drivenwheel on the high coefficient-of-friction side is above a firstthreshold value, checking whether the slip of the driven wheel on thelow coefficient-of-friction side is above a second threshold value,checking whether an acceleration of the traction slip or a wheelacceleration of the driven wheel on the high coefficient-of-frictionside is above a positive threshold value and whether the acceleration ofthe traction slip or the wheel acceleration of the driven wheel on thelow coefficient-of-friction side is below a negative threshold value,and checking the brake pressures on the two wheels or comparing thebrake pressures one to the other, further including the step ofdecreasing the brake pressure according to a definable gradient. 19.Method as claimed in claim 18, wherein the gradient is determinedaccording to the acceleration of the traction slop of the wheel on thehigh coefficient-of-friction side or the wheel on the lowcoefficient-of-friction side.
 20. Device for controlling a traction slipof a vehicle on a roadway with sidewise different coefficients offriction, comprising: an identification device for identifying a drivingsituation on a roadway with sidewise different coefficients of friction,a brake actuation control which decreases the brake pressure of thedriven wheel on the low coefficient-of-friction side when the drivingsituation is identified and traction slip is encountered on both wheels,wherein the identification device includes the following components: adetermining device for determining which vehicle side is the highcoefficient-of-friction side and which is the lowcoefficient-of-friction side, and a checking device for checking whetherthe slip of the driven wheel on the high coefficient-of-friction side isabove a first threshold value, whether the slip of the driven wheel onthe low coefficient-of-friction side is above a second threshold value,whether an acceleration of the traction slip or the wheel accelerationof the driven wheel on the high coefficient-of-friction side is above apositive threshold value, and whether the acceleration of the tractionslip or the wheel acceleration of the driven wheel on the lowcoefficient-of-friction side is below a negative threshold value, andfor checking the brake pressures on the two wheels or comparing thebrake pressures one to the other, wherein that when comparing the brakepressures a check is made as to whether the brake pressure on the highcoefficient-of-friction side is below the brake pressure on the lowcoefficient-of-friction side at least by a defined amount.
 21. Devicefor controlling a traction slip of a vehicle on a roadway with sidewisedifferent coefficients of friction, comprising: an identification devicefor identifying a driving situation on a roadway with sidewise differentcoefficients of friction, a brake actuation control which decreases thebrake pressure of the driven wheel on the low coefficient-of-frictionside when the driving situation is identified and traction slip isencountered on both wheels, wherein the identification device includesthe following components: a determining device for determining whichvehicle side is the high coefficient-of-friction side and which is thelow coefficient-of-friction side, and a checking device for checkingwhether the slip of the driven wheel on the high coefficient-of-frictionside is above a first threshold value, whether the slip of the drivenwheel on the low coefficient-of-friction side is above a secondthreshold value, whether an acceleration of the traction slip or thewheel acceleration of the driven wheel on the highcoefficient-of-friction side is above a positive threshold value, andwhether the acceleration of the traction slip or the wheel accelerationof the driven wheel on the low coefficient-of-friction side is below anegative threshold value, and for checking the brake pressures on thetwo wheels or comparing the brake pressures one to the other, whereinthe brake actuation control includes a preset values output forpredefining a nominal gradient for brake pressure decrease.
 22. Deviceas claimed in claim 21, wherein the preset values output determines thegradient according to the slip or the acceleration of the traction slipof the wheel on the high coefficient-of-friction side or the wheel onthe low coefficient-of-friction side.